It has been over sixty years since the first use of replacement parts for hip joints. There have been many advances in the prosthetic components, materials, surgical techniques and the like, so that total hip joint replacement has become relatively commonplace. Related techniques have also been used for replacing knee and shoulder joints.
There are two principal components to a hip replacement prosthesis. One is an acetabular cup which is implanted in the acetabulum. The acetabular cup provides a spherical socket which is the bearing surface for the replacement joint. The other component comprises a femoral stem which is fitted into the medullary canal of the femur and a femoral head on the steam having a spherical surface which meets with the acetabular socket.
The femoral portion of the prosthesis is inserted by cutting off the femoral neck with or without removing the greater trochanter. The medullary canal is then prepared using drills, reamers and successively larger rasps to produce a cavity which is closely complementary to the femoral stem. After cleaning, the femoral stem is driven into place in the canal with what is essentially a press fit. Preparing the cavity to fit the stem is tedious and prolongs the period the patient must be kept under anaesthesia.
The femoral stem may be held in place by a polymethylmethacrylate cement (PMMA) or it may be provided with a porous surface on the shank which accommodates ingrowth of cancellous bone which secures the femoral component in the femur.
The acetabular cup is implanted after grinding a socket in the pelvis to receive it. The acetabular cup may be secured with cement, or may be fastened to the bone with screws after a press fit. Similar techniques, differing in detail are used for implanting replacement shoulder joints, knees and the like.
Despite advances in the technology of hip replacement, it is found that a substantial number of "revisions" are required. Such revisions involve removing components of the hip joint and replacing them. Such revisions may be required shortly after the original surgery due to complications. More commonly they occur eight or ten years after the original surgery due to any of a number of problems that may arise. Such revisions are traumatic for the patient, tedious for the surgeon, and quite time consuming for surgical staff and facilities.
A principal problem in revisions is removal of the femoral component. Some such components are made with transverse holes or threaded holes for connection of tools to extract the femoral stem from the medullary canal. Repeated hammer blows may be applied for driving the stem out of the cavity. Sometimes a window is cut in the femoral cortex near the distal end of the shank, and a punch and hammer are used for driving the shank toward the open end of the femur. Trauma to the patient can be severe and breakage of parts of the femur is not unusual. The techniques employed for removing the femoral component have been characterized as barbaric.
Another technique that has been attempted is removal of the polymethylmethacrylate with an ultrasonically vibrated osteotome. Such a technique is described in U.S. Pat. No. 4,248,232 by Engelbrecht. The osteotome is used for scooping out polymethylmethacrylate cement softened by the ultrasonic vibrations.
Other techniques involve use of long, thin osteotomes for cutting either the cement used for securing the prosthesis in the medullary canal or cancellous bone in the case of an ingrowth prosthesis. In effect, the osteotomes are long chisels which are tapped to disintegrate the cancellous bone or cement and free the prosthesis from the surrounding cortex. For example, in a paper entitled "Atraumatic Removal of a Well-Fixed Porous Ingrowth Hip Prosthesis, " Orthopedic Review, Vol 15, No. 5, Jun. 1986, page 387, Doctors McClelland, James and Simmons describe removal of a femoral component "by the use of an oscillating saw and long, thin osteotomes to carefully separate the prosthesis from its intra-medullary environment. This portion of the procedure was both tedious and somewhat time-consuming, but no iatrogenic damage to the cortical tube of the proximal femur resulted. After the proximal half of the prosthesis had been freed up in this manner, the prosthesis was then extractable, using multiple heavy hammer blows applied to vise grips attached to the end of a McReynolds-wedge extractor."
Another difficulty encountered during revision surgery involves removal of the cement "plug" from the medullary canal. Typically, the initial cavity formed in the medullary canal is deeper than the length of the femoral portion of the prosthesis. The area surrounding the femoral stem and between the distal end of the femoral stem and the end of the medullary canal is filled with cement during initial implantation.
Prior art methods for removing the cement plug from the distal end of the canal include drilling through the cement to either tap the hole with a female thread or to engage a hook around the rear surface of the plug. However, the relative hardness of the cement compared to the bone creates a risk that the drill will migrate to the side and perforate the bone. In addition, progress of the drill is generally observed using x-ray visualization.
It is clear that faster and less traumatic techniques are desirable for removing components of prostheses inserted in the medullar canal. In addition, there remains a need for an improved method of removing the cement plug from the distal end of the medullary canal which reduces the risk of wall perforation and which can be accomplished without the use of x-ray visualization.